FIG. 6 is a schematic diagram illustrating a prior art semiconductor laser device. In FIG. 6, reference numeral 101 designates a heat sink. A laser element 102 emitting laser light is mounted on a part of the heat sink 101. A photodetector 103 for monitoring the intensity of the laser light is disposed on a part of the heat sink 101 spaced from the laser element 102. Reference numerals 104, 105, and 106 designate voltage applying terminals. When a voltage sufficient to operate the laser element is applied across the terminals 104 and 105, laser light beam 107 and 108 are emitted from opposite facets of the laser. The laser element 102 and the photodetector 103 are positioned so that the photodetector 103 is irradiated with laser light beam 108. When a voltage sufficient to operate the photodiode 103 is applied across the terminals 104 and 106, current flowing through the photodetector 103 is measured, whereby the intensity of the laser light 108 from the laser element 102 is monitored and, synchronously, the intensity of the laser light 107 is monitored indirectly.
FIG. 7 is a sectional view of the laser element 102 shown in FIG. 6, illustrating the laser light emitting facet. In FIG. 7, reference numeral 1 designates an n type GaAs substrate. An n type AlGaAs lower cladding layer 2 is disposed on the GaAs substrate 1. A p type AlGaAs active layer 3 is disposed on the lower cladding layer 2. A p type AlGaAs upper cladding layer 4 is disposed on the active layer 3. The upper cladding layer 4 has a stripe-shaped ridge 6 serving as a current path in the center of the structure. N type GaAs current blocking layers 5 are disposed on the upper cladding layer 4, contacting the opposite sides of the ridge 6. A p type GaAs contact layer 7 is disposed on the current blocking layer 5 and on the stripe-shaped ridge 6. An anode electrode 9 is disposed on the contact layer 7, and a cathode electrode 10 is disposed on the rear surface of the substrate 1.
A description is given of the operation.
When a plus voltage is applied to the anode electrode 9 and a minus voltage is applied to the cathode electrode 10, since the pn junction between the active layer 3 and the lower cladding layer 2 is biased in the forward direction, a forward current flows toward the pn junction. The current is concentrated in the ridge structure 6 by the current blocking layers 5. In FIG. 7, arrows 13 show the current flow. Therefore, the current is concentrated in a central region of the active layer 3 directly under the ridge 6, and laser oscillation occurs in that region. A dotted circle 14 shows a spatial distribution of laser light. A part of the laser light reaches the GaAs current blocking layer 5 and is absorbed because the band gap energy of the GaAs current blocking layer 5 is smaller than the band gap energy of the AlGaAs active layer 3. As a result, laser oscillation occurs in a transverse mode that is parallel with the active layer 3, i.e., that is stable in the horizontal direction.
In the laser device show in FIGS. 6 and 7, since the external photodetector 103 is necessary for monitoring the intensity of laser light emitted from the laser element 102, the cost of assembling of the photodetector 103 is considerable.